1. Field of the Invention
This invention relates to repair of high pressure turbine shrouds. More particularly, it relates to the method of repairing high pressure turbine shrouds utilizing a high velocity oxyfuel (HVOF) and materials used for such repairs.
2. Discussion of Prior Art
In gas turbine engines, a shroud typically surrounds the tips of the rotor blades in the turbine section of the engine. Pressurized air and fuel are burned in a combustion chamber to add thermal energy to the medium gases flowing therethrough. The effluent from the chamber comprises high temperature gases, which are flowed downstream in an annular flow path through the turbine section of the engine. Nozzle guide veins at the inlet to the turbine directed the medium gases onto a multiplicity of blades which extend radially outward from the engine rotor. An annular shroud that is supported by the turbine case surrounds the tips of the turbine blades to contain the medium gases flowing thereacross to the flow path. The clearance between the blade tips and the shroud is minimized to prevent the leakage of medium gases around the tips of the blades. Shrouds provide a rubbing surface for the tip of the blade. The design intent is for the blade tip to rub into the shrouds, thus reducing the amount of air that can bypass the turbine airfoils. Minimizing the air that can bypass the turbine airfoils increases the efficiency of the engine. A secondary function of the shroud is to thermally shield the case from the hot flow path gas.
The shroud thus is exposed to abrasion from the rotating turbine blade tips. Simultaneously, the shroud also is exposed to the hot flow path gases that are burned in a combustion chamber. These gases over a period of time not only result in corrosion and high temperature oxidation of the shroud, but also function to cause erosion of the shroud surfaces. Thus, the shroud must be designed to be at once resistant to corrosive and oxidation effects of the hot gases, erosion resistant to the constant flow of the hot gases over the shroud surfaces and abrasion resistant, or rub compliant, as a result of the contact with the turbine blade seal teeth.
Over a period of time, as the engine it utilized, the surfaces of the shrouds tend to be worn from the rubbing surfaces of the blades' tips. In addition, some erosion takes place as the hot gases mechanically erode the shroud flow path surfaces. Additionally, some corrosion and oxidation of the shroud surfaces also occurs due to the corrosive action of the gases on the shroud surfaces.
Because of the high cost of the shroud materials, rather than dispose of the shrouds that are made from expensive superalloy material and machined to exacting and tight tolerances, it is desirable to repair the shrouds by restoring the shrouds to their original dimensions in accordance with preselected tolerances as determined by the engine's size as well as to restore the corrosion resistant properties to the flow path surfaces. In the past, this restoration has been accomplished by low pressure plasma spray (LPPS) or by use of thermally densified coatings (TDC). While both of these methods provide repairs and restorations that are effective, both suffer from some limitations. For example, the VPS and LPPS processes spray MCrAlY in a vacuum chamber on a heated substrate, making the process very sensitive to leaks, as the partial vacuum must be maintained in order to successfully accomplish the repair. Only a limited number of parts can be processed at any one time with the LPPS process. Additionally, LPPS requires a preheat, and coupled with the welding process, can result in considerable part distortion. While this method has the advantage of being able to provide a repaired shroud that can be used at higher temperatures than other methods, the deposition of the material also is accomplished at a much slower rate. The result is than the shroud is either restored to minimum or below minimum dimensions, or significant cost is incurred in adding additional material to the shroud during repair. The result is that this method is slow, time consuming and considerably expensive. The TDC process utilizes brazed preforms, which may be in the form of powders, to build up the sides and the flow paths on all the surfaces. The preforms typically include epoxy as a bonding agent. The result is that the parts typically include some undesirable, and sometimes unacceptable porosity. Of course, the quality of parts repaired by the TDC process is dependent on the quality of the preforms. The materials that are utilized in a TDC process typically contain melting point depressants such as silicon and boron or combinations of these elements. Because these materials are designed to melt at temperatures of about 2300.degree. F. or less, they must be applied at temperatures below the incipient melting temperature of the base material. Shrouds that are repaired using these materials cannot be utilized in applications above about 2250.degree. F.
What is desired is a method of repairing high pressure turbine shrouds after engine running to extend the life of the shrouds and provide cost effective operation of the engine while applying oxidation-resistant, corrosion-resistant and rub-compliant materials that can withstand temperatures higher than about 2250.degree. F.